Graphene is a single atom thick planar sheet of sp2-bonded carbon atoms which are positioned in a honeycomb crystal lattice. The term “graphene” is also used to represent structures having a small number of graphene layers and similar properties. The distinctive structure of graphene confers many unique mechanical, electronic, thermal, optical and magnetic properties upon it, in addition to quantum effects that have never been found in other materials. For example, the charge carriers in graphene behave as massless Dirac fermions and present an ambipolar field effect and room-temperature quantum Hall effect.
Graphene has remarkably high electron mobility, up to 2×105 cm2V−1s−1 at room temperature. This is due to the ease with which electrons can move through the lattice, it being free of imperfections and heteroatoms. Its thermal conductivity is also remarkably elevated and was recently measured as high as 3000 wm−1k−1, larger than those measured for carbon nanotubes and diamond. This combination of properties makes graphene a promising candidate to take the place of Si as a new generation of materials in the semiconductor industry. Graphene may also have widespread potential applications in electronics and optoelectronics such as field-effect transistors, light-emitting diodes, solar cells, sensors and panel displays.
It has been both theoretically predicted and experimentally proved, that size, composition and edge geometry of graphene are important factors, which determine its overall electronic, magnetic, and optical and catalytic properties due to strong quantum confinement and edge effects. For example, by cutting graphene sheets to long and narrow ribbons (GNR) (width less than 10 nm) it is possible to induce a direct band gap in graphene, that renders GNRs semiconducting (M. Y. Han, et al. Phys. Rev. Lett., 2007, 98, 206805-2066808). Further confinement in the basal plane (overall dimensions smaller than 100 nm) leads to quantum dots (GQDs) with zero dimensions. The suppressed hyperfine interaction and weak spin-orbit coupling make GQDs interesting candidates for spin qubits with long coherence times for future quantum information technology (A. Donarini, et al. Nano Lett., 2009, 9, 2897-2902.). Therefore graphene sheets with reduced lateral dimensions in the form of nano-ribbons or quantum dots can effectively tune the bandgap of graphene and facilitate the lateral scaling of graphene in nanoelectronic devices. In this context it has become urgent to develop effective routes for tailoring the graphene structures (J. Lu, et al. Nat Nanotechnol. 2011, 6, 247-252, L. A Ponomarenko, et al. Science, 2008, 320, 356-358).
Currently, there are a number of possible methods by which graphene sheets may be fabricated, which include chemical vapour deposition (CVD), micromechanical cleavage, epitaxial growth and chemical exfoliation. Compared with other techniques, chemical exfoliation, which involves the direct exfoliation of various solid starting materials, such as graphite oxide, expanded graphite and natural graphite, is advantageous in terms of simplicity, cost and high volume production. However, currently explored chemical solution exfoliation methods have a number of drawbacks that need to be addressed.
The most commonly used chemical exfoliation method employs the chemical oxidation of graphite to negatively charged graphite oxide sheets, which can be readily exfoliated as individual graphene oxide sheets by ultrasonication in water. To restore graphene's unique properties the oxygen containing groups are removed by chemical reduction; however without the charges, the strong Van der Waals interactions among the reduced graphene sheets result in their immediate coalescence and restacking. Very recently it was found that the addition of ammonia in the aqueous solution can lead to stable aqueous dispersions of graphene because of the electrostatic repulsion from the negatively charged carboxylic acid groups that remain on the surface of the sheets. Other attempts to prevent graphene aggregation have mainly focused on coating the graphene oxide surface with a dispersant phase, usually a surfactant or a polymer resulting in weak internanosheet repulsions.
The addition of foreign molecules to graphene is, however, undesirable for many applications and leads to the graphene produced by these chemical exfoliation methods being quite poor in quality compared to that fabricated by CVD and micromechanical cleavage. This is mainly because the various chemicals used, such as solvents, oxidants and reductants may attack the graphene lattice in the process or are difficult to be removed, leading inevitably to residual surface species. Overall these chemical processes introduce various forms of surface defects, which disrupt the graphene band structure and hamper the conductivity of the resulting graphene sheets. New strategies to produce relatively clean graphene sheets in bulk quantity while keeping them individually separated are required
Another disadvantage of the known chemical exfoliation methods is that many of the chemicals used are either expensive or toxic and need careful handling, leading to environmentally unfriendly and unsustainable approaches. Furthermore, the majority of chemical solution exfoliation methods involve extremely time-consuming multiple steps that sometimes last for several days. For example, the oxide defects present in graphene oxide can be removed by thermal, or a combination of chemical and thermal, reduction which adds another step in the processing procedure. In addition, thermal reduction is most successfully carried out at ˜1000° C., a temperature which is unsuitable for many applications.
Alternative processes, which overcome the above mentioned obstacles, and allow for the formation of high-quality graphene, have therefore been investigated. To date, some progress has been achieved.
Recently Coleman and co-workers from Trinity College Dublin demonstrated (Nature Nanotechnology 3 563, 2008) that it is possible to exfoliate graphite to produce single- and few layer graphene by judiciously choosing a solvent which ensures a strong interaction between solvent and graphene surface. However the yield of this process is small and not appropriate for mass scale production. Direct exfoliation of graphene in organic solutions improves the yield, but this is achieved only following prolonged sonication times approaching 3 weeks in duration or extended ultracentrifugation.
Liu et al. reported in Chem. Commun., 2010, 46, 2844-2846, that single layered and bilayered graphene sheets can be produced by a direct exfoliation from graphite flakes in the presence of single stranded DNA using a simple sonication. Production of graphene sheets from graphite by sonication in ionic liquids has also been reported by Wang et al. (Chem Commun., 2010, 46, 2844-2846), Nuvoli et al. (J. Mater. Chem., 2010) and in WO 2010/065346. However, the graphene sheets produced by these simple techniques still contain a few impurities (such as fluorine, sulphur etc.), and a large fraction of oxygen (more than 10 at %) similar to that found in graphene reduced from graphene oxide. Oxygen in graphene is difficult to be removed and may significantly influence the property and application of graphene. Thus, an alternative process capable of making graphene sheets of high quality and high concentration is highly desirable for high volume production.
There remains, therefore, a need for new solution-phase methods to produce significant quantities of high quality (low in defects or defect free, and/or unoxidized or substantially non-functionalised) graphene. In particular, new processes which possess improvements in cost and efficiency and/or which are capable of producing graphene of higher quality or in higher yields than those known in the art are needed. Improvement in multiple factors is desirable. Ultimately, a commercial process which can be used to make graphene on a large scale is desired. The present inventors have surprisingly established that the production of high quality graphene is possible using a processes involving the mixing and grinding of natural graphite with one or more ionic liquids. The use of natural graphite can not only decrease the cost compared to the expanded graphite or graphene oxide, but also improve the quality of resulting graphene due to the low oxygen content. Such processes can produce graphene in higher yields than was previously possible and hence represent promising alternatives to those already known in the art. In particular, the processes of the invention address the issue of providing solvent free, low impurity graphene and may be suitable for use on a large scale. Furthermore, the use of ionic liquids is potentially less expensive and more environmentally acceptable than the previously developed alternatives.